This invention concerns a device for testing the integrity of a bond in a semi-conductor device, and more particularly the strength of a bond between a substrate and a means of electrical connection thereto, typically a part-spherical deposit of solder or gold.
Semiconductor devices are very small, typically from 0.2 mm square to 15 mm square. These devices have sites for the bonding of electrical conductors thereto Sites typically comprise part spherical deposits of gold or solder, collectively known as balls, which in use have the appearance of a low circular dome, and a diameter in the range 50-1000 xcexcm. These deposits form part of the electrical path between, for example, a printed circuit board and a chip, and may directly connect components, or may be joined to a wire which is itself connected to another component.
In the case of gold, the ball may comprise the balled end of a gold wire which is joined to an electrical path by a thermocompression technique. In the case of solder, a discrete ball may be applied to a substrate and reflowed to form a semi-circular bump for subsequent connection to another component.
It is necessary to test the mechanical strength of the interermetallic bond between the gold or solder deposit and the substrate in order to give confidence that the bonding method is adequate, and that the bond strength is sufficient. Difficulties arise because of the very small dimensions of the components, the precision with which the testing device must be positioned, and the very small forces and deflections which are to be measured.
It has been proposed to test such deposits by applying a flat shear tool to one side thereof. In order to avoid friction caused by the tool rubbing on the surface of the substrate, it is necessary for the tool to be just above the substrate surface. The height of the tool must be closely controlled to give accurate force measurement, typically within xc2x10.001 mm. The flat shear tool typically makes an initial point contact with the deposit, and that contact may be poor due to the curvature of the deposit. The initial contact may result in deformation of the deposit due to the high point load, and this may cause a mechanical failure of the deposit before a significant load is applied to the bond. Breakage of the deposit may be due to cutting in by the shear tool. If this happens information concerning the ultimate bond strength cannot be obtained.
The gold ball provides a particular problem because alternative techniques, such as a tensile test using mechanical tweezers, are not possible due to the very small diameter of a typical gold deposit.
What is required is a better means for shear testing which can overcome the aforementioned problems, particularly those problems associated with the very small diameter of a typical gold deposit. The principal objective is to maximise the load which can be applied to the intermetallic interface.
According to the invention, there is provided a test device for shear testing of electrical conduction deposits of semiconductor devices, said deposits being dome-like and having a diameter in the range 50-1000 xcexcm, said test device comprising a test head having a recess adapted to closely engage said deposit over part of the circumference thereof for the application of a shear force.
Typical deposits are soft electrical conduction materials such as solder and gold.
The recess may closely engage the deposit over part of the surface thereof, for example a part-spherical surface.
Preferably the recess is semicircular and cylindrical. Such a recess is relatively easy to manufacture compared with, for example, a part spherical recess. In any event the test head should engage the deposit over sufficient area to ensure that shearing at the bond site is the expected mode of failure. In general the closer the recess approximates to the shape of the deposit, the more likely is shear failure at the interface between the deposit and the substrate. However if shear failure at the bond interface can be achieved using a tool of simple form, a more complex form may not be necessary. What is required will depend on the type of material to be tested and the nature of the bond at the inter metallic interface; this can be determined empirically using techniques to be described later in the specification.
In a preferred embodiment the recess is a part-cylindrical, preferably semi-cylindrical cavity having a diameter within which will closely fit at least 75% and most preferably 95% of all deposits to be tested. Such cavity ensures that reshaping of oversize or undersize deposits is minimised. However gold and solder are relatively soft materials, and a small amount of reshaping can be accommodated without significant alteration of the bond interface. Preferably reshaping of the deposit occurs over 30% or less of the circumference of the deposit. In the case of very precisely formed deposits, the recess can closely approach the ideal diameter for a line contact or a narrow band contact with minimal reshaping.
A small amount of reshaping may be beneficial in that the deposit can more closely conform to the shape of the recess. Such reshaping is preferably limited to a depth of less than 10% of the diameter of the deposits, and most preferably less than 5%.